Change minimal Python required version to 3.7

Drop python 3.6 support from v0.3.0, following
[JAX deprecation policy](https://jax.readthedocs.io/en/latest/deprecation.html).

PiperOrigin-RevId: 410793443

setup.py

@@ -62,7 +62,7 @@ def _parse_requirements(requirements_txt_path):
     extras_require=_EXTRA_PACKAGES,
     tests_require=_parse_requirements(
         os.path.join(_CURRENT_DIR, 'requirements_tests.txt')),
-    requires_python='>=3.6',
+    requires_python='>=3.7',
     include_package_data=True,
     zip_safe=False,
     # PyPI package information.